You don't say what exactly you are looking for with regards to information about your grandfather. A good place to start with is probably his birth certificate. This will list the names of his parents at the very least. You can get the birth certificate from the local registry office in Pontevedra.
Once you have that, depending on the brevity of the document, you might have an address or at least the area of Pontevedra where his parents lived. You can then find the local parish where he would have been baptised. Church records often times have far more information than registry documents. The record comes in form of a book, which you may be able to peruse at your leisure. This means you can go back and forwards a few years to see whether your grandfather had any siblings (which seems likely given your aunt's trips there). You might need to go in person though...
Finding descendants of those siblings will be a bit harder, but could be as simple as a local phone book if they still live in the area.
As for the language barrier. I find GoogleTranslate gets you a long way. The translations are usually good enough to be understood, even if they can't get nuance across as well. If you would rather have a human look over it, you could try the genealogy reddit or Discord server. Folks there are usually very friendly and happy to help.